The present invention relates to a construction of a direct current motor, especially to a preferable construction of a direct current motor having a rotating speed which can be varied over a wide range.
As is well known in the art, armature windings of a direct current motor, which is disposed in the magnetic flux generated by field magnets, are supplied with electric current and, then, the armature is rotated by the cooperation of the armature current and the magnetic field. The direction of the current supplied to the armature is commutated in accordance with the rotation of the armature by utilizing a commutator and brushes.
When the armature of such a direct current motor is supplied with armature current, magnetic fields are generated around the armature by the armature current. The magnetic fields generated by the armature current have various influences on the magnetic flux generated by the field magnets, which are generally called armature reactions. Such armature reactions are, for example, (a) displacement of the electrically neutral axis, (b) decrease of the magnetic flux and (c) nonuniformity of voltage on the commutator and local high voltage on the commutator.
One of the armature reaction phenomena is that the magnetic flux generated by the armature current displaces the electrically neutral axis from the geometric neutral axis defined by the field magnets, as mentioned in item (a), above. When the electrically neutral axis is displaced, magnetic flux can exit on the geometric axis. When the above-mentioned magnetic flux is interlinked with the armature windings, voltage is induced in the armature windings. As a result, when a brush is disposed on the geometric neutral axis, the induced voltage is shorted and a spark is generated. As a result, a large electric current flows to the brush and commutation becomes difficult; in other words, the commutating characteristics are degraded.
To obviate the degradation of the commutating characteristics, an interpole is usually disposed at a position between two adjacent field magnets. However, in some cases, for example in a situation wherein the required characteristics of the motor do not permit leakage of magnetic flux through the interpole, it is difficult to dispose the interpole so as to obviate the degradation of the commutating characteristics.